One difficulty lies in effectively controlling phase and amplitude distortions of antennae and RF chains in substantially “open-loop” operation as, apart from the loss of the signal upon reception at satellites, there is not really a way of monitoring the quality of the emitted signal, and when the loss arises, it is not known a priori how to state which radiating element of the antenna is defective.
These problems are particularly pertinent for ground stations as, with the antenna being in open sky, temperature effects may affect the controlling of the radiation patterns, and in the context of SDMA multiple access (and not as ordinary TDMA access with beamforming each time), since all of the signals are then emitted at the same time. Ultimately, the radiation pattern has to be controlled up to a low elevation, which requires a high degree of control thereof.
To manage this, it is generally required to measure the radiation patterns of the radiating elements of the active antenna, before it is deployed, in a controlled laboratory environment, in an anechoic chamber.
The main difficulty is that making tests in an anechoic chamber are not necessarily representative, especially for active antennae equipped with a large number of radiating elements, typically greater than 20, which therefore require very large anechoic chambers, notably in order to take account of the effects of pressure and of temperature on them.
These solutions entail a high cost, not only for using an anechoic chamber, but also for choosing technologies that are relatively insensitive (for example to temperature) in order to facilitate this calibration.